A description is provided here below of the frequency resources of equipment of a network working especially according to a Wi-Fi technology as defined in the IEEE 802.11r standard proposing a fast BSS transition of a station between a first set of basic services managed by a first access point and a second set of basic services managed by a second distinct access point.
According to this standard, an access point manages a unique given channel. A mechanism is proposed in order to enable a switching of a data transmission from one access point to another access point and, therefore, in general, with a change in channel in the event of mobility of the terminal. In other words, this IEEE 802.11r standard enables a switching of a data transmission, the concerned terminal then having to get associated with a new access point managing a new channel.
This switching mechanism comprises several lengthy and complex processing phases. Indeed, the switching terminal must “discover” the new access point, get authenticated and get associated with this access point, especially by exchange and recognition of encryption keys.
Despite the gains provided by this technique where the duration of preparation of the transition (up to a few seconds) is anticipated, there remains nevertheless a duration of transition of the order of 50 ms. Such a switching duration introduces a momentary interruption of service, which is not acceptable for numerous applications, and for example for the transmission of a video.
In the event of disturbance of the radio channel by a radar, it has been envisaged that an access point can change channels in the same frequency band, to avoid disturbing the radar transmissions considered to be priority transmissions. Indeed, in this case, a channel switching can be implemented by a same access point. To this end, the 802.11h standard specifies especially adaptations for the management, in Europe, of the spectrum and of the transmission power of computer networks using a wireless link in the 5 GHz frequency band. The standard stipulates in particular mechanisms of dynamic selection of frequency and control of the transmission power also known as “Spectrum and Transmit Power Management Extensions in the 5 GHz band”.
In particular, in this standard, dynamic frequency selection (DFS) is based on a frame for managing the switching which makes a “channel switch announcement”. Thus, an access point deciding to launch a channel switching procedure, sends this specific management frame with an access to the priority channel (channel free for a specific duration denoted as “PIFS” for “point coordination function inter-frame spacing”). This frame contains especially the number of the new channel and the countdown time before the change. This change is expressed in number of beacons.
A beacon is a management frame containing all the pieces of information of the communications network, especially the pieces of information on the channel on which they are transmitted periodically by the access point. They play a stamping role, and synchronize all the terminals of a same channel, attached to the access point. The beacon intervals between the sending of two successive beacons are parametrized by the access point. Besides, these beacons contain pieces of information making it possible to know the characteristics of the basic service sets proposed by the access point, for example the identity of the access point (BSSID or “Basic Service Set Identification”), the frequency band, the channel number in this frequency band, and the options supported by the PHY/MAC layers.
This standard offers the advantage of interrupting the transmissions for a very short time, since the stations recover the same access point in the new channel, with no fresh authentication or fresh association. However, this approach enables only a change of channel in a same frequency band.
Moreover, this approach is not a multi-channel approach, since an access point is always, at a given point in time, associated with a single channel. Indeed, the drawback of this technique according to the 802.11h standard is that it gives rise to the channel switching of all the terminals attached to the access point. In other words, if a channel changing action is launched, the totality of the flows managed by the access point is rerouted to another channel. It is not possible to effect a distribution of the load between different channels for a considered access point.
The American patent application US 2004/185887 concerns a wireless network node in a multi-channel context, the network comprising at least two transceivers statically tuned to non-interfering channels. The drawback of this technique lies in the fact that, at each change of channel, an association and authentication procedure has to be performed, consequently stretching the transmission interruption time during the channel switching or band switching.
There is therefore a need for a solution enabling the acceleration of these changes of channels for multi-channel access points with a view to current projects. Indeed, a situation of asynchronous channels managed by different mechanisms is currently envisaged in the future IEEE 802.11ac standard currently being prepared.
This future standard targets the use of increasingly broad radiofrequencies channels having for example a width equal to 80 MHz (corresponding to the aggregation of four 20 MHz channels) or more.
Thus, FIG. 1a illustrates this mechanism for a desired channel with a width of 20 MHz, 40 MHz, 60 MHz or 80 MHz, capable of being constituted, in one alternative, by four adjacent 20 MHz channels. To this end, this mechanism defines a primary channel on which there is applied a CSMA-CA (“Carrier Sense Multiple Access-Collision Avoidance”) access mode as described in the 802.11-2007 standard, paragraph 9.1 “MAC architecture” and 9.1.1 “DCF” as well as the sending of a beacon by an access point further managing a channel known as a secondary channel, a tertiary channel and a channel called a quaternary channel. In this alternative, the CSMA-CA access mode is implemented only on the primary channel, the other three channels being synchronized on this primary channel, i.e. working according to the same access mode.
However, this future standard stipulates that it can prove to be difficult to find a free 80 MHz band in the available band and proposes an alternative, illustrated in FIG. 1b, to transmit at 80 MHz in creating two channels desynchronized and separated in frequencies which, when added together, form an 80 MHz channel. Thus, in this alternative shown in FIG. 1b, the two desynchronized channels are 40 MHz channels, each formed by a primary channel (1 and 2) and a secondary channel (1 and 2). On each of the primary channels 1 and 2, two distinct access modes and two distinct beacons are then used, the entire set being managed by a single access point then possessing a multi-channel function.
Besides, the future IEEE 802.11ad standard, also under preparation, pertains to a Wi-Fi system working at 60 GHz and introduces the possibility of switching between the 60 GHz frequency band and the 5 GHz frequency band. This future standardization also requires a fast and fluid switching without deterioration of a possible transmission in progress, whether or not the 60 GHz and 5 GHz channels are synchronized.
In this particular context pertaining to future standardizations, the inventors have therefore identified a need for a novel technique for optimizing the allocation of resources and the operations of switching from one channel to another, fluidly and rapidly, whether the channels are synchronous (i.e. managed by the same access mode) or asynchronous (i.e. managed by access modes distinct from one channel to another), and whether or not the channels are contiguous.